The present invention relates to a thin film semiconductor device and a semiconductor substrate sheet to be used in the semiconductor device as well as a method for producing them.
As is well known, a thin film semiconductor device (also known as a thin film transistor (TFT) device) is formed on a semiconductor substrate, which typically consist of a thin film layer of semiconductor material, such as silicon, over a base layer of insulation material, such as non-alkaline glass, or quarts glass. In the thin film layer of the semiconductor, a plurality of channels consisting of a source area and a drain area are formed, and each of channels is equipped with a gate electrode separated by an insulating film from the above areas.
In a typical thin film semiconductor device, a gate electrode insulator is interposed between the gate electrode and the channel area. This insulator is usually formed with a film of silicon oxide, and this film is typically required to be formed at a low temperature. To form this silicon oxide film in a TFT, high temperature silicon oxide film formation techniques such as those used in large scale integration (LSI) semiconductor processes (which may require temperatures of more than 900° C.) typically cannot be used. Instead, a relatively low temperature deposition process (e.g., a temperatures less than 600° C.), such as one using a plasma CVD method, is used.
Although an oxidized film deposited by the plasma CVD method can be used to form an insulator film for a TFT, it may have disadvantages in insulation property and/or stability compared with a film oxidized at a high temperature. This occurs because, when using the plasma CVD-method, some impurities remain between the channel area and the gate insulator film, and further, the resultant silicon oxide film has a tendency to be composed of compounds as which do not have a stoichiometrically regulated composition of “SiO2” but, instead, have an irregular composition such as “SiO1.9”. When an oxidized film having such characteristics is used as a gate electrode insulator of a TFT, the TFT circuit tends not only to have greater variance of threshold voltage values, but also have reduced long-term stability of TFT properties. For example, in conventional products, variation of TFT threshold voltage values may be in the ±0.4V range and the magnitude of this variation may increase over time.
Furthermore, in conventional thin film semiconductor devices using poly-crystalline silicon, disadvantages due to the small size of crystal grains and the irregularity of configuration mode of crystal grains are inevitable. That is, as a poly-silicon film is composed of a numerous crystal grains of extreme small size, the improvement of mobility is limited due to such phenomenon as dispersion of electrons or holes at boundaries between crystal grains at the time of operation of the device.
Attempts have been made to use relatively large grain size polycrystalline silicon in order to avoid or minimize disadvantages such as electron dispersion while simultaneously maintaining high electron mobility. For example, thin film layers having semiconductor grains of about 1 μm size and having a mobility of about 100 cm2/V sec. have been formed by annealing a layer of polycrystalline silicon in a high temperature furnace. However, high temperature annealing (e.g., over 1000° C.) is used in this process, and this requires the use of expensive quartz glass sheets instead of relatively inexpensive sodium glass sheets. A substrate using such expensive materials is not suited for cost-effective production of a device using a large substrate, such as a TFT LCD display screens.
Other processes to obtain a thin layer film of polycrystalline semiconductor having large size grains have been proposed. These methods include irradiating a thin film of amorphous or polycrystalline semiconductor with an energy beam (such as an excimer laser) instead of using high temperature annealing. By this irradiation method, it is possible to enlarge the size of a crystal grain using relatively inexpensive glass sheets as the base layer. However, even excimer laser irradiation is used, the size of obtained crystal grain generally does not exceed 1 μm. Furthermore, this excimer laser process can cause unevenness of grain size. Incidentally, the grain size can be determined by “(the maximum diameter of a grain+the minimum diameter of the grain)÷2” and such diameters can be measured through SEM observation of crystal grains which remains after etching the film by Secco etching process.
Furthermore, there is a problem on the configuration of crystal grains in a thin film formed using the conventional excimer laser process. Namely, in the conventional polycrystalline semiconductor thin film, configuration of crystal grains in the two dimensional direction may be highly random. The random configuration of crystal grains, and the non-uniformity of grain size, may cause serious difficulty in forming TFT devices. Such difficulties may occur because electron mobility may fluctuate when a device is formed traversing the border of crystal grains and, therefore, it is difficult to integrate TFT circuits having different channel lengths.
Accordingly, many TFT on the market, in which poly-crystalline semiconductor film are used, are designed so that one circuit extends so as to contain at least several boundaries of crystal grains as mentioned in FIG. 7, in order to reduce the variation of mobility. In such devices, the average mobility cannot exceed 150 cm2/V·sec.